The present invention relates to a catalytic component for the polymerization of olefins combining a number of catalytic components for the polymerization of olefins, and to a process for preparing it. The catalytic component according to the invention is capable of being obtained by impregnation of a prepolymer with a solution of a catalytic component for the polymerization of olefins. The prepolymer obtained by prepolymerization of at least one olefin in the presence of the catalytic component according to the invention is also a subject of the present invention. The polymer obtained by polymerization of at least one olefin in the presence of the prepolymer according to the invention or of the catalytic component according to the invention is also a subject of the present invention.
Catalytic component is understood to mean, generally and in particular in the present application, an ingredient of a catalytic system for the polymerization of olefins, the said ingredient containing a transition metal and the said transition metal being active for the polymerization of olefins. The transition metal is known as active for the polymerization of olefins when the catalytic component which contains it catalyses the polymerization of lefins, if appropriate also by virtue of the presence in the polymerization medium of other ingredients of the said catalytic system. By way of example, the titanium contained in a conventional catalytic component of Ziegler-Natta type containing titanium, chlorine and magnesium atoms is the transition metal and is active for the polymerization of olefins. In fact, this type of catalytic component catalyses the polymerization of olefins by virtue of the titanium and also by virtue of the presence in the polymerization medium of a cocatalyst, in general an organic aluminium derivative, which is another ingredient of the catalytic system of which the catalytic component containing the titanium forms part.
The term prepolymerization is used to denote polymerization leading to a prepolymer. Prepolymer is generally understood to mean the active hydrocarbon solid or polymer obtained by prepolymerization of at least one olefin in the presence of a catalytic component, this solid representing not more than ten per cent of the mass of the polymer which it is finally desired to synthesize by polymerization of at least one olefin in the presence of the said prepolymer. Generally, the kinetics of prepolymerization are controlled by controlling the flow rate of the olefin(s) intended to be prepolymerized. Generally, the degree of progression of the prepolymerization is less than or equal to 10,000 grams of prepolymer per millimole of active transition metal contained in the catalytic component from which the prepolymer has resulted. This degree of progression is calculated by taking into account the total polymer mass contained in the prepolymer. Thus, if the catalytic component contains a polymer, the degree of progression is calculated by determining the sum of the polymer mass formed during the prepolymerization and the polymer mass contained in the starting catalytic component and by then dividing this sum by the number of moles of transition metal contained in the starting catalytic component. The use of the term prepolymer necessarily implies that the prepolymer under consideration is active for the polymerization of olefins, that is to say that it catalyses the polymerization of olefins, if appropriate in the presence of a suitable cocatalyst, which can be of the same nature as the cocatalyst used during the polymerization which has led to the prepolymer.
The combination of a number of catalytic components for the polymerization of olefins has already been attempted. The document EP 0,439,964 A2 describes the preparation of solid catalytic components containing both a component based on Mg, Cl and Ti and a derivative of a transition metal, the said metal having a cycloalkadiene group for ligand. This type of component leads to polymers with broadened molecular mass distributions.
U.S. Pat. No. 5,032,562 describes the preparation of polyolefins with multimodal molecular mass distributions by virtue of the use of a catalytic component comprising a magnesium derivative containing Mgxe2x80x94OR bonds, a zirconium derivative and TiCl4.
Patent Application EP 447,070 A1 teaches that a catalytic component prepared by impregnation of MgCl2 with two electron donors, then addition of a zirconium metallocene and then addition of TiCl4 leads, on polymerization, to a polymer with the bimodal molecular mass distribution.
Document EP 452,920 teaches that it is possible to carry out a prepolymerization in the presence of a catalytic component containing both a transition metal bonded to a group with the cyclopentadienyl skeleton non-bridged to another group with the cyclopentadienyl skeleton and a compound containing an Alxe2x80x94O bond, and then to bring the prepolymer thus obtained into contact with a derivative of a transition metal comprising at least two ligands each comprising a cyclopentadienyl skeleton, the two ligands being connected to each other by a divalent radical. Polymerization of olefins in the presence of the product thus obtained has a narrow composition distribution and a good melt strength.